This invention relates to water recovery and, in particular, to water recovery for use in high temperature fuel cell systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions. High temperature fuel cells, such as molten carbonate fuel cells, operate by passing a reactant fuel gas through the anode, while oxidant gas comprising carbon dioxide and oxygen is passed through the cathode.
Reactant gases supplied to the fuel cell, and in particular, the reactant fuel gas supplied to the anode, must be sufficiently humidified to maintain a desired steam to carbon ratio in the fuel for proper and efficient fuel cell operation and to prevent carbon deposition in the fuel cell. The amount of water consumed by fuel cell system for humidifying reactant gases is usually significant and requires a continuous supply of water to the fuel cell system.
The electrochemical reaction between the reactant fuel gas and the oxidant gas produces water vapor outputted as part of a heated anode exhaust gas, which also comprises excess fuel not consumed by the fuel cell and CO2. In order to limit or eliminate water supplied to the fuel cell system from external sources, it is desired to separate water vapor in the anode exhaust gas from other anode exhaust components and to recycle the separated water to humidify the reactant gases. Water recovery from the system exhaust is possible through the use of condensing heat recovery heat exchangers. However, the amount of water recovered will depend on the local ambient temperature and humidity levels, and the recovered water is usually insufficient to support fuel cell operation in many high temperature locations. Water recovery from the more humid anode exhaust gas is usually more efficient over a wider range of ambient temperatures.
An example of such water recycling is disclosed in U.S. Pat. Nos. 5,068,159 and 5,039,579, which teach using a cooler and condenser to separate water from the anode exhaust stream, and thereafter passing the separated water through a boiler and a heater and feeding the water to the inlet of the anode compartment.
Another U.S. Pat. No. 7,060,382, assigned to the same assignee hereof, discloses a system in which a water transfer assembly in the form of a partial-pressure swing water transfer wheel is used to separate and transfer water vapor in anode exhaust as water to the fuel feed. This patent also discloses a system in which this transfer is carried out by using heat exchangers where the anode exhaust is cooled by the oxidant supply gas, water recycle vaporization and/or a cooling water or ambient air-cooled heat exchanger (air fan). After being cooled, the stream is fed to a scrubbing and blow-down assembly where the electrolyte contaminated water is removed. The resultant stream is then further cooled in a heat exchanger by an air fan or cooling water and the stream then fed to a condensing unit. At this unit, the water is removed and fed to the fuel feed, while the stream is further fed to a carbon dioxide transfer assembly.
Conventional systems for separating water in the anode exhaust employ complex and costly cooling systems with heat exchangers to achieve sufficient cooling of the hot anode exhaust gas leaving the anode in order to condense a sufficient amount of water from the anode exhaust. This results in significant power consumption, greatly increasing the operating costs of the system. The costs of cooling the anode exhaust gas to separate the water from the other anode exhaust components is particularly significant in warmer and more humid climates, in which anode exhaust must be cooled below ambient temperature in order to obtain sufficient condensation and a sufficient amount of water for use in humidifying the reactant gases.
It is therefore an object of the present invention to provide an improved water recovery assembly capable of separating and transferring water from anode exhaust gas without requiring significant power for operation.
It is another object of the present invention to provide a water recovery assembly which produces water free of electrolyte, thus eliminating the need for a water treating system.
It is a further object of the present invention to provide a water recovery assembly which results in greater manufacturing and operating efficiencies in the fuel cell system.